For reasons such as enhanced operational efficiency and environmental considerations, there are air-conditioning apparatuses that perform air conditioning by using a zeotropic refrigerant mixture in which a plurality of refrigerants with different boiling points are mixed. For example, in some conventional air-conditioning apparatuses using a zeotropic refrigerant mixture such as multi-air-conditioning apparatuses for buildings, a composition-sensing bypass circuit is added to the main refrigerant circuit to sense the composition (circulating composition) of a zeotropic refrigerant mixture circulating through the refrigerant circuit. For example, the composition-sensing bypass circuit is formed by composition-sensing heat exchangers and an expansion device, with temperature and pressure sensors attached in the flow path. A part of the refrigerant discharged from the compressor is caused to flow through the composition-sensing heat exchanger (high-pressure side), the expansion device, and the composition-sensing heat exchanger (low-pressure side) in the order named, and bypassed to the suction portion (suction-side pipe) of the accumulator. At this time, the temperature of the refrigerant in a supercooled liquid state (supercooled liquid refrigerant) at the outlet of the composition-sensing heat exchanger (high-pressure side), the temperature of the refrigerant that is in a two-phase state (two-phase refrigerant) after passing through the expansion device, and the pressure (low-pressure side pressure) at the suction portion of the accumulator are detected by the temperature and pressure sensors. Then, the circulating composition is computed on the basis of the temperature of the supercooled liquid refrigerant, the two-phase refrigerant temperature, and the low-pressure side pressure (see, for example, Patent Literature 1).